Proofreading exonuclease activity of human DNA polymerase δ and its effects on lesion-bypass DNA synthesis

Replicative DNA polymerases possess 3′ → 5′ exonuclease activity to reduce misincorporation of incorrect nucleotides by proofreading during replication. To examine if this proofreading activity modulates DNA synthesis of damaged templates, we constructed a series of recombinant human DNA polymerase δ (Pol δ) in which one or two of the three conserved Asp residues in the exonuclease domain are mutated, and compared their properties with that of the wild-type enzyme. While all the mutant enzymes lost more than 95% exonuclease activity and severely decreased the proofreading activity than the wild-type, the bypass efficiency of damaged templates was varied: two mutant enzymes, D515V and D402A/D515A, gave higher bypass efficiencies on templates containing an abasic site, but another mutant, D316N/D515A, showed a lower bypass efficiency than the wild-type. All the enzymes including the wild-type inserted an adenine opposite the abasic site, whereas these enzymes inserted cytosine and adenine opposite an 8-oxoguanine with a ratio of 6:4. These results indicate that the exonuclease activity of human Pol δ modulates its intrinsic bypass efficiency on the damaged template, but does not affect the choice of nucleotide to be inserted.     

Antioxidant activity in the leaves of <Emphasis Type="Italic">Melilotus albus</Emphasis> and <Emphasis Type="Italic">Trifolium medium</Emphasis> from man-made disturbed habitats in the Middle Urals under the influence of copper

Physiological mechanisms of adaptation to copper-induced stress in two widespread legume plants, white sweet clover (Melilotus albus Merik.) and zigzag clover (Trifolium medium L.), growing in habitats differing in the man-made pollution. An antioxidant plant defense system was activated in response to 10 mM CuSO₄, which is a stress factor. Specific biochemical features related to adaptation to soil contamination with copper were observed in tested plant species. Superoxide dismutase was activated in response to stress in both species from various habitats. M. albus from the impact zone manifested the better capacity of proline accumulation as compared with plants from less polluted habitats. T. medium plants from the impact zone contained more active peroxidase. It was suggested that plants growing for a long time under stressful conditions manifest the greater tolerance to copper ions than plants, which did not experience stress or were subjected to the milder stress.